The transition from the expression of surface immunoglobulin to the active secretion of antibody marks the ultimate stage in the differentiation of B lymphocytes. This transition is reflected in the virtually exclusive expression of secreted mu (mus) mRNA over membrane mu (mum) and delta mRNAs. Stage-specific termination of transcription between the C mu and C delta gene segments is known to be the mechanism which controls delta heavy chain expression in terminally differentiated antibody- secreting cells. Our studies now indicate that the sequences involved in stage-specific transcription termination are closely linked to the membrane mu sequences involved in the polyadenylation of mum mRNA. This proposal is directed at determining the elements and mechanisms involved in stage- specific transcription termination in the mu and delta complex transcription unit. The close proximity of the region of stage- specific termination to the signals for mum mRNA polyadenylation may reflect a linkage of these processes involved in regulating B lymphocyte maturation.